The Long Term Evolution (LTE) standard has been extended with support of device to device (D2D) (specified as “sidelink”) features targeting both commercial and Public Safety applications. Applications include device discovery, where devices are able to sense the proximity of another device and associated application by broadcasting and detecting discovery messages that carry device and application identities. Another application includes direct communication based on physical channels terminated directly between devices.
A potential extension of the D2D features is vehicle-to-“other” communication (V2x). V2x communication, also referred to as vehicle-to-vehicle/infrastructure or vehicle-to-anything imaginable, includes any combination of direct communication between vehicles (V2V), pedestrians (V2P), and/or infrastructure (V2I). V2x communication may take advantage of a network infrastructure, when available, but at least basic V2x connectivity should be possible even in case of lack of coverage. Providing an LTE-based V2x interface may be economically advantageous because of the LTE economies of scale, and it may enable tighter integration between communications with the network infrastructure (e.g., V2I) and V2P and V2V communications, as compared to using a dedicated V2x technology.
V2x communications may carry both non-safety and safety information, where each of the applications and services may be associated with specific requirements sets, e.g., in terms of latency, reliability, capacity, etc. ETSI has defined two types of messages for road safety: the Co-operative Awareness Message (CAM) and the Decentralized Environmental Notification Message (DENM).
The CAM message is intended to enable vehicles, including emergency vehicles, to notify their presence and other relevant parameters in a broadcast fashion. Such messages target other vehicles, pedestrians, and infrastructure, and are handled by their applications. CAM messages also serve as active assistance to safety driving for normal traffic. The availability of a CAM message is indicatively checked for every 100 ms, yielding a maximum detection latency requirement of less than 100 ms for most messages. However, the latency requirement for a pre-crash sensing warning is 50 ms.
The DENM message is event-triggered, such as by braking, and the availability of a DENM message is also checked for every 100 ms. The requirement of maximum latency is less than 100 ms.
The package size of CAM and DENM messages varies from 100+ to 800+ bytes, and the typical size is around 300 bytes. The message is supposed to be detected by all vehicles in proximity.
The SAE (Society of the Automotive Engineers) has also defined the Basic Safety Message (BSM) for Dedicated Short Range Communications (DSRC), with various message sizes defined. According to the importance and urgency of the messages, the BSMs are further classified into different priorities.
In cases of high user density, the contention-based spectrum used by V2x UEs may become highly fragmented. This may prevent high efficiency in the resource utilization in cases where high resource load needs to be accomplished. An existing solution is to use Decentralized Congestion Control (DCC) algorithms (such as, for example, ETSI TS 102 687) in high resource load situations. DCC algorithms reduce the load by means of increasing the transmit data rate or by reducing the packet rate. Similarly, DCC algorithms use power control to reduce interference. These methods, however, may not reduce spectrum fragmentation, or do so at the expense of reducing the number of transmissions (i.e., reducing the packet rate). Thus, there is a need for an improved manner of handling high resource load situations that reduces resource fragmentation.